Is artificial intelligence vulnerable to hacking?
AI is vulnerable from two security perspectives the way I see it: 


The classic method of exploiting outright programmatic errors to achieve some sort of code execution on the machine that is running the AI or to extract data.
Trickery through the equivalent of AI optical illusions for the particular form of data that the system is designed to deal with.


The first has to be mitigated in the same way as any other software. I'm uncertain if AI is any more vulnerable on this front than other software, I'd be inclined to think that the complexity maybe slightly heightens the risk.

The second is probably best mitigated by both the careful refinement of the system as noted in some of the other answers, but also by making the system more context-sensitive; many adversarial techniques rely on the input being assessed in a vacuum.